The known optical memory cards provided approx. 4-6 MB of data on a credit card sized optical card, and the reader/write units offer 30-10 KB/s data transfer rate. Writing speeds are normally slower than reading speeds.
Holographic recording is known for its inherent high data density, and therefore has been suggested for use in a data storage card. A number of solutions have been proposed for incorporating holograms into data storage cards, but the holograms is mostly used for authentication purposes, and not for data storage. Using holograms to store data on a credit-card sized data card involves several problems. First, most holographic techniques require that the photosensitive medium storing the hologram be illuminated from both sides, either during recording or during readout. Accordingly, the recording medium, preferably an optical card holding the holograms should have an optical-quality surface on both sides, and have constant thickness. These requirements are difficult to meet with a conventional plastic card. Second, for data storage application it is desirable to use a recording medium that may be erased and re-recorded. There are very few erasable optical materials that are suitable for holographic recording, the achievable signal-to-noise ratio is relatively low and high exposition energy is needed. Third, with every readout, the recorded holograms will be slightly erased. To ensure the stability of the recorded holograms, different reading and writing wavelength is required, but in this case the reconstructed image of the hologram is distorted so much, that high-density storage is not possible.
A known method of reflection holography is disclosed in the publication DE 195 34 501 A1., and in the publication “High density disc storage by multiplexed microholograms”, SPIE Vol. 3109, pp. 239-244. In these solutions a method is suggested to create reflection holograms. It is suggested to apply a mirror under the recording layer during the recording phase, so that the object beam reflected from the mirror will act as reference beam. Thereby no separate optical path is needed for the reference beam. It is suggested to multiply the storage capacity by different forms of multiplexing. The holograms are reconstructed as volume reflection holograms. A disadvantage of the proposed solution is that the mirror must be removed during readout, which makes this system unfit for practical optical recording systems. Also, there is no suggestion to use this method with an optical card.
Another form for reflection holography is disclosed in the U.S. Pat. No. 5,633,100, which patent teaches a process for forming a volume reflection holograms. This known solution also require the use of a reference beam that is incident on the opposite surface of the photosensitive medium, so the solution is not practical for an optical card. U.S. Pat. No. 4,888,260 discloses another method for the preparation of a volume phase reflection hologram. Here, the volume phase reflection hologram is formed by a second off-axis hologram in the same recording medium. This method is not suitable for forming erasable and re-recordable holograms, and the optical system is very complicated. U.S. Pat. No. 5,710,645 discloses a method and system for recording a grazing incidence hologram, which is supported on a substrate having a thin edge-illuminatable geometry, like an optical card. Theoretically, this system could be used for data storage as well, but again the edge-illumination demands very special mechanical and optical properties of the card carrying the hologram.
The document “Side-chain liquid crystalline polyesters for optical information storage”, published in OPTICS LETTERS, vol. 17. no. 17. September 1992, pages 1234-1236, New York, US mentions the possibility of polarisation holographic recording in combination with different write and read laser wavelength. However, this document does not address the problem of distortion caused by the difference in the wavelengths.
The document WO-A-97/02563 discloses an optical system for holographic recording. This known system also includes lasers with a different read and write wavelength. The suggested data storage medium is a card with a thick (50 μm) holographic storage layer. Different forms of holography are suggested, but polarisation holography is not mentioned. The write and read optics contain waveguide structures in combination with detector cells to read out the data, instead of traditional optical systems. The optical head detects the intensity modulation caused by the recorded holograms directly, and there is no imaging system between the storage medium and the optical head. Therefore, the problem of wavelength distortion is not addressed either. On the other hand, the disclosed complex waveguide head comprises expensive acousto-optical elements and other electro-optical devices which require very sophisticated control and power supply systems. This optical head can not be manufactured in a cost-effective way with current technology.
Therefore, it is the object of the invention to provide a method and system for data storage based on reflection holography, where the holograms may be recorded and erased several times, preferably in an unlimited number of cycles, and where the holograms need to be accessed from one side only, both during writing and readout. Also, the holograms should be stored on an optical recording medium, preferably an optical card or disk that is easily to manufacture, and which tolerates normal daily wear, i.e. which is subjected to the same or similar treatment as a traditional plastic credit card or a floppy disk. It is a further object of the invention to provide a method and system for data storage where the read-write apparatus contains a relatively small, simple and cheap optical system. It is a further object to provide an optical recording method that ensures high data density and high data transfer rate, and at the same time allow efficient encoding or encryption of the data, and thereby provides enhanced security.